1. Field of the Invention
The present invention relates to a positive electrode for lithium battery and to a lithium battery using it.
2. Description of the Related Art
Conventionally, for an electric power source for electronic instruments, a manganese dioxide-zinc battery is used as a primary battery and a nickel battery such as a nickel-cadmium battery, a nickel-zinc battery, and a nickel-hydride battery, and a lead battery are used as a secondary battery. As the electrolyte for these batteries, alkaline solution such as potassium hydroxide or acid solution of sulfuric acid is used.
Recently, with the progress of electronic instruments, a new high-performance battery has been expected and the development of a high energy density battery using a non-aqueous electrolyte has been proceeded in place of the above-described aqueous electrolyte battery. As the typical battery, there is a lithium battery using lithium or a carbon material as a negative electrode.
The lithium battery includes a primary battery such as a manganese dioxide-lithium battery and fluorinated carbon-lithium battery and a secondary battery such as a manganese dioxide-lithium battery, a vanadium oxide-lithium battery, etc.
The secondary battery using metallic lithium as the negative electrode has a disadvantage that a short-circuit is apt to occur by the dendritic growth of metallic lithium deposited at charging and the life is short. Also, there is a problem that it is difficult to ensure the safety thereof owing to the high reactivity of metallic lithium. Thus, a so-called lithium ion battery using graphite, carbon or the like as the negative electrode to prevent the dendritic deposition of the metallic lithium by forming an intercalation compound thereof with lithium and using lithium cobaltate as the positive electrode has been devised and the battery is used as a high energy density battery.
However, because lithium cobaltate is expensive, a lithium-containing manganese composite oxide or lithium nickelate is proposed for lithium cobaltate. In the case of the lithium-containing manganese composite oxide, there is a problem that the theoretical capacity is low and also with the progress of the charge-discharge cycle, the reduction of the obtained capacity becomes large.
On the other hand, lithium nickelate has the same crystal structure as lithium cobaltate but has a fault that when charging and discharging are repeated, the capacity is decreased. Recently, there is provided an attempt of synthesizing lithium nickelate exhibiting a homogeneous charging-discharging reaction by lithium nitrate and nickel oxyhydroxide containing cobalt. However, in any case, to make a positive electrode using lithium nickelate, a process of grinding the course particles after synthesis to form fine particles, mixing the fine particles with carbon, etc., as an electro-conductive material, and coating the mixture together with a binder such as polyviylidene fluoride and the like on to a metal current collector such as aluminum, nickel and the like is required. Accordingly, there is a problem that the production process is complicated and also the performance is strongly influenced by the amounts of the carbon powder and the binder.
Furthermore, there is an attempt of making a positive electrode for a lithium battery using nickel oxyhydroxide which is an utterly different active material from the above-described active materials. For example, in Unexamined Japanese Patent Publication (kokai) No. Sho-63-19760, it is proposed to use nickel oxyhydroxide containing from 20 to 75% cobalt and in Unexamined Japanese Patent Publication (kokai) No. Sho-63-19761, it is proposed to use nickel hydroxide charged in an aqueous solution of lithium hydroxide as an active material, but they are insufficient in the performance and have not yet been practically used up to now.
As described above, nickel oxyhydroxide has the same layer structure as lithium cobaltate practically used but in spite of that nickel oxyhydroxide is proposed in Unexamined Japanese Patent Publication (kokai) No. Sho-63-19760, it has not been practically used up to now. When the cause is considered from the view point of electrode reaction, it is considered to be caused by that the diffusion of lithium ion into the positive electrode active material with the charging-discharging reaction, that is, the intercalation of the lithium ion into nickel oxyhydroxide does not uniformly occur. Also, it is considered to be one of the causes that the optimum electrode structure of the active material and the current collector has not yet been established.
Aluminum is mainly used for the material of a current collector which is used for a positive electrode of a lithium battery at present. For the nickel hydroxide positive electrode of a nickel-cadmium battery and a nickel-hydride battery, a sintered nickel substrate or a three-dimensional nickel porous material serving as a current collector is used as an active material holding material but it has scarcely been investigated to use nickel as the material of a positive electrode for a lithium battery which is a nonaqueous electrolyte battery. The reason is that not only nickel is dissolved in an electrolyte but also it is feared that nickel is deposited on a negative electrode to cause the short-circuit of the battery. Also, because LiCoO.sub.2, LiNiO.sub.2, V.sub.2 O.sub.5, LiMn.sub.2 O.sub.4, etc., which are general as positive electrode active materials used for lithium batteries are particles of about 20 .mu.m average diameter produced by burning usually at a temperature of 500.degree. C. or higher called a solid phase method, it is particularly impossible, in fact, to apply these particles to a sintered nickel substrate having an average pore diameter of about 10 .mu.m.